While staying with his weird great-aunt Kathryn, Evan visits an eerie old toy store and buys a dusty can of Monster Blood. It's fun to play with at first. And Evan's dog, Trigger, likes it so much, he eats some. But then Evan notices something weird about the slimy green ooze. It keeps growing. And growing. And growing. And all that growing has given the Monster Blood a monstrous appetite. . . .